Assume that the assemblage described above is a combiner having six input ports receiving corresponding signals which are transferred from such ports to the common port to provide therefrom an output combining such originally separate signals. In the case of, say, the signal which is received by the #1 input port and conducted in a path #1 from such port towards the output port, a fraction of that signal will, on its reaching the end of the #1 path, be diverted through the output port to become a desired component of the composite output signal. Since, however, at such end of that #1 path, the other five input ports are electrically coupled in parallel with the output port, other fractions of the #1 signal will, unless something is done, reach such other input ports to there be manifested as extraneous signals. The presence of such extraneous signals at such ports is undesirable because they may flow reversely through such ports, and because of other detrimental electrical effects likely to be produced.
It has been proposed in an article "A New N-Way Power Divider/Combiner Suitable for High Power Applications" authored by Ulrich H. Gysel and published in the MIT Symposium Digest, 1975, pages 116-118 that such problem may be overcome as follows. As disclosed in that article, a microwave circuit (which will be assumed to be a combiner circuit) comprises circuit boards and, also, transmission lines which are all in the form of striplines printed on such boards except that one of such lines is a coaxial line. In such circuit, a primary port is connected by a coaxial line Z.sub.1 to a junction to which are also connected a plurality of striplines Z.sub.2 connected at their ends away from such junction to corresponding secondary ports. The lines Z.sub.2 provide principal paths for transfer of microwave signals between the secondary ports and the mentioned junction.
In order for a signal received at any one secondary port to reach through principal paths any other secondary port as an extraneous signal, that signal must travel through two principal paths a distance between those two ports which is a half wavelength of the microwave signal at the midfrequency of the combiner. The results is that such extraneous signal undergoes approximately a 180.degree. phase shift in the course of such travel.
To reduce the presence of the extraneous signals at the secondary ports, these ports are respectively connected to a plurality of supplemental signal transfer paths each consisting of a stripline Z.sub.3 and a stripline Z.sub.4 in series, and all connected to a common floating point at their ends away from the secondary ports. Each of such supplemental paths has a length of one half wavelength. Because of the existence of these supplemental paths, the signal received at any one secondary port can reach any other secondary port as an extraneous signal not only through two principal paths as described above but also through two supplemental paths. However, the fraction of that signal which travels through the two supplemental paths to the destination port undergoes in the course of such travel a phase shaft of 360.degree. so as to be exactly out of phase with the fraction of extraneous signal reaching that port through the two principal paths. Hence, if the extraneous signal fractions reaching that destination port through, respectively, the two principal paths and the two supplemental paths are of about the same amplitude (as can be realized), these two signal fractions will almost wholly cancel each other out so as to reduce to low level the resultant extraneous signal at that port.
The circuit disclosed by the Gysel article has, however, the disadvantages that, because of the several odd impedance transmission lines required, stripline or microstrip construction is indicated. However, for high power combining of larger numbers of signals with minimum loss, stripline does not work well, and the circuit is undesirably limited as to the microwave power it can handle as a result of the lower power carrying capacity of the striplines.
One or more of the disadvantages just mentioned of the Geysel circuit are overcome according to the invention which is disclosed and claimed in my U.S. Pat. No. 5,223,809 for "Signal Isolating Microwave Splitters/Combiners" issued Jun. 29, 1993 and assigned to the assignee hereof. The exemplary embodiment of the invention disclosed in that earlier filed application is a microwave assemblage of a configuration enabling it to incorporate therein a number of secondary ports which can vary in number to six or to even more than six, that particular embodiment having six of such ports. Often, however, when only two, three or four of such ports are required, the spatial features of that assemblage which are used in it to accommodate a larger number of secondary ports are not needed, and the presence of those features in the configuration causes it to occupy space which might well be put to better purposes. Moreover, such assemblage is of costly construction because its structure incorporates a large number of expensive junction boxes.